Radio communication device and radio communication method

ABSTRACT

Provided is a radio communication device which can separate propagation paths of antenna ports and improve a channel estimation accuracy even when using virtual antennas. The device includes: a mapping unit ( 103 ) which maps a data signal after modulation to a virtual antenna ( 0 ) and a virtual antenna ( 1 ); a phase inversion unit ( 104 ) which inverts the phase of S 0  transmitted from an antenna port ( 2 ) in synchronization with a phase inversion unit ( 105 ) between the odd-number slot and the even-number slot; the phase inversion unit ( 105 ) which inverts the phase of R 0  transmitted from the antenna port ( 2 ); a phase inversion unit ( 113 ) which inverts the phase of S 1  transmitted from an antenna port ( 3 ) in synchronization with a phase inversion unit ( 114 ); and the phase inversion unit ( 114 ) which inverts the phase of R 1  transmitted from an antenna port ( 3 ).

CROSS-REFERENCE(S) TO RELATED APPLICATION(S)

The present application claims priority under 35 U.S.C. 119(e) ofJapanese Patent Application No. 2007-213077, filed on Aug. 17, 2007, andJapanese Patent Application No. 2008-163032, filed on Jun. 23, 2008, thedisclosures of which are incorporated herein by reference including thespecifications, drawings and abstracts in their entirety.

TECHNICAL FIELD

The present invention relates to a radio communication apparatus and aradio communication method.

BACKGROUND ART

3GPP-LTE adopts OFDMA (Orthogonal Frequency Division Multiple Access) asa downlink communication scheme. According to 3GPP-LTE, a radiocommunication base station apparatus (hereinafter abbreviated as a “basestation”) transmits a reference signal (“RS”) using predeterminedcommunication resources and a radio communication terminal apparatus(hereinafter abbreviated as a “terminal”) performs channel estimationusing the received reference signal and demodulates data (see Non-PatentDocument 1).

Furthermore, when the base station is provided with a plurality ofantenna ports, the base station can carry out diversity transmission. Onthe other hand, in order for a terminal to receive adiversity-transmitted signal without errors, the terminal needs to knowthe conditions of the propagation path from the antenna port group usedin the base station to the terminal. Therefore, RSs need to betransmitted from all antenna ports provided in the base station withoutmutual interference. To realize this, 3GPP-LTE adopts a method oftransmitting RSs using different timings and carrier frequencies in thetime domain and the frequency domain from individual antenna ports inthe base station.

FIG. 1A illustrates a configuration of a base station having two antennaports (2-Tx base station) assumed in 3GPP-LTE and FIG. 1B illustrates anRS transmission method by a 2-Tx base station. Likewise, FIG. 2Aillustrates a configuration of a base station having four antenna ports(a 4-Tx base station) assumed in 3GPP-LTE and FIG. 2B illustrates an RStransmission method by a 4-Tx base station. In FIG. 1B and FIG. 2B, thevertical axis (frequency domain) shows subcarrier units and thehorizontal axis (time domain) shows Orthogonal Frequency DivisionMultiplexing (OFDM) symbol units. Furthermore, one slot is made up ofseven OFDM symbols. Furthermore, R0, R1, R2 and R3 indicate The RSstransmitted from antenna ports 0, 1, 2 and 3 (the first, second, thirdand fourth antenna ports). Furthermore, a unit of one block enclosed bya frame of a bold line (12 subcarriers in the frequency domain, sevenOFDM symbols in the time domain) will be referred to as a “resourceblock (“RB”).” As is clear from FIG. 1B and FIG. 2B, the 4-Tx basestation reduces the frequency of RS transmission from antenna port 2(third antenna port) and antenna port 3 (fourth antenna port) tominimize the overhead on RS transmission.

By the way, a 1-Tx base station transmits RS using the only resources ofR0 in the RS arrangement by the 2-Tx base station.

As described above, the 4-Tx base station has a low transmissionfrequency of RSs from antenna port 2 and antenna port 3. Therefore, aterminal that receives RSs from the 4-Tx base station cannot interpolatethe channel estimate values of antenna port 2 and antenna port 3 in oneRB, and, consequently, the accuracy of channel estimation isdeteriorated severely during high-speed movement. Therefore, it has beenconfirmed that avoiding the use of antenna port 2 and antenna port 3 ofthe base station during high-speed movement of the terminal can improvethe SNR performance at the terminal (see Non-Patent Document 2).

Therefore, although a 4-Tx base station has been conventionally providedwith four antenna ports, only two antenna ports are used when a terminalis moving at high speed.

Alternatively, in order to use four radio transmitting sections of a4-Tx base station effectively, as shown in FIG. 3, conventionally, a4-Tx base station is conventionally handled as a virtual 2-Tx basestation provided with virtual antenna 0 made up of antenna port 0 andantenna port 2 and virtual antenna 1 made up of antenna port 1 andantenna port 3. However, in FIG. 3, a CDD (Cyclic Delay Diversity)generation section is added to antenna port 2 and antenna port 3 tosuppress unnecessary beam forming effect caused by virtual antennas.

In this case, assuming that a signal outputted from the mapping sectionin FIG. 3 is:

$\begin{matrix}{S_{2{Tx}} = \begin{pmatrix}s_{0} \\s_{1}\end{pmatrix}} & \lbrack 1\rbrack\end{matrix}$the signal y_(virtual) transmitted from the four antenna ports isrepresented by:

$\begin{matrix}{y_{virtual} = {{D\begin{pmatrix}1 & 0 \\0 & 1 \\1 & 0 \\0 & 1\end{pmatrix}}s_{2{Tx}}}} & \lbrack 2\rbrack\end{matrix}$where D is a 4×4 diagonal matrix representing CDD.

-   Non-Patent Document 1: 3GPP TS 36.213 V1.1.0, R1-072633.-   Non-Patent Document 2: Transmit Diversity Scheme for Control Channel    in E-UTRA, R1-072423.

DISCLOSURE OF INVENTION Problems to be Solved by the Invention

However, when virtual antennas such as the ones described above areused, since a base station transmits the same RS from two antenna ports,a terminal cannot separate between the propagation paths of four antennaports in the base station. For this reason, the terminal cannot know theconditions of the propagation paths between individual antenna ports andthe terminal. Therefore, it is no longer possible to optimize spacedivision multiplexing (“SDM”), which is made possible when the basestation controls the transmission weights for the four antenna portsindividually. This problem has a significant impact on a terminal movingat low speed (hereinafter abbreviated as a “low-speed terminal”).

Furthermore, the terminal cannot know the number of antenna ports at abase station until a BCH (Broadcast CHannel) signal is received fromthat base station. According to 3GPP-LTE, since a 1-Tx base station isalso present, a terminal, not knowing the number of antenna ports at abase station, cannot help but perform channel estimation using only R0that is sure to be transmitted. Therefore, when a base station having aplurality of antenna ports transmits a BCH signal using antenna portsother than antenna port 0, the transmission method of the base stationand the reception method of a terminal do not match. Therefore, when abase station having a plurality of antenna ports transmits a BCH signalusing only antenna port 0, the base station cannot perform diversitytransmission of the BCH signal despite having a plurality of antennaports. Therefore, the coverage of a BCH signal is smaller than thecoverage of a data signal capable of diversity transmission.

It is therefore an object of the present invention to provide a radiocommunication apparatus and a radio communication method capable ofseparating between the propagation paths of a plurality of antenna portsand improving the accuracy of channel estimation even when using virtualantennas.

Means for Solving the Problem

The radio communication apparatus according to the present inventionadopts a configuration including a virtual antenna formed with aplurality of antenna ports and an inversion section that inverts a signof one of reference signals transmitted from the plurality of antennaports respectively.

The radio communication method according to the present invention is aradio communication method for a radio communication apparatus having avirtual antenna formed with a plurality of antenna ports, the methodinverting a sign of one of reference signals transmitted from theplurality of antenna ports respectively.

Advantageous Effects of Invention

According to the present invention, it is possible to separatepropagation paths of a plurality of antenna ports and improve channelestimation accuracy even when using virtual antennas.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1A is a block diagram illustrating a configuration of aconventional 2-Tx base station;

FIG. 1B illustrates an RS transmission method in the conventional 2-Txbase station;

FIG. 2A is a block diagram illustrating a configuration of aconventional 4-Tx base station;

FIG. 2B illustrates an RS transmission method in the conventional 4-Txbase station;

FIG. 3 is a block diagram illustrating a configuration of a conventionalvirtual 2-Tx base station;

FIG. 4 is a block diagram illustrating a configuration of a base stationaccording to Embodiment 1 of the present invention (when operating as avirtual 2-Tx base station that carries out virtual antennatransmission);

FIG. 5 is a block diagram illustrating a configuration of the basestation according to Embodiment 1 of the present invention (whenoperating as a 4-Tx base station that carries out antenna porttransmission);

FIG. 6 illustrates an RS arrangement according to Embodiment 1 of thepresent invention;

FIG. 7 illustrates an arrangement of a BCH and an SCH in the time domainaccording to 3GPP-LTE;

FIG. 8 illustrates an arrangement of data channels and control channelsof a BCH and an SCH in the frequency domain according to 3GPP-LTE;

FIG. 9 is a block diagram illustrating a configuration of a base stationaccording to Embodiment 2 of the present invention;

FIG. 10A illustrates an RS arrangement according to Embodiment 2 of thepresent invention (the frequency band not including the BCH);

FIG. 10B illustrates an RS arrangement according to Embodiment 2 of thepresent invention (the frequency band including the BCH);

FIG. 11 illustrates an RS arrangement of a conventional 1-Tx basestation;

FIG. 12 is a block diagram illustrating a configuration of a basestation according to Embodiment 3 of the present invention (whenoperating as a virtual 4-Tx base station that carries out virtualantenna transmission);

FIG. 13 is a block diagram illustrating a configuration of the basestation according to Embodiment 3 of the present invention (whenoperating as an 8-Tx base station that carries out antenna porttransmission); and

FIG. 14 illustrates an RS arrangement according to Embodiment 3 of thepresent invention.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter embodiments of the present invention will be described indetail with reference to the accompanying drawings.

Embodiment 1

With the present embodiment, a 4-Tx base station transmits RSs and datasignals using two virtual antennas formed with two antenna ports each(virtual antenna transmission). However, the sign of a signaltransmitted from one of the antenna ports forming one virtual antenna isinverted per RB in the time domain.

The base station uses virtual antennas in this way and therefore canoptimize transmission quality of a signal directed to a terminal whichis moving at high speed (hereinafter abbreviated as a “high-speedterminal”) while using four antenna ports effectively. Furthermore, theterminal can separate RSs from the two virtual antennas into RSs fromthe four antenna ports, which allows the terminal to perform channelestimation for all antenna ports.

Furthermore, with the present embodiment, the base station transmits adata signal to a terminal suitable for transmission of data signalsusing the four antenna ports, a low-speed terminal in particular, usingthe four antenna ports without using virtual antennas (antenna porttransmission). However, for antenna port 2 and antenna port 3, it ispreferable to transmit data signals via a CDD generation section addedat the time of formation of the virtual antenna.

This allows a terminal desiring virtual antenna transmission to bepresent with a terminal desiring antenna port transmission within a cellcovered by a base station.

Furthermore, according to the present embodiment, a base station mayalso transmit RSs and data signals while inverting the signs of virtualantennas in the frequency domain, not in the time domain.

This allows the terminal to average the channel estimate values obtainedfrom RSs in the time domain (before separation), so that it is possibleto improve the accuracy of channel estimation at the terminal.

Furthermore, according to the present embodiment, a 4-Tx base stationcontinues virtual antenna transmission as a virtual 2-Tx base stationunless requested otherwise from a terminal. That is, a 4-Tx base stationswitches virtual antenna transmission to antenna port transmissionaccording to request from a terminal. In this way, the presentembodiment assumes virtual antenna transmission as the basictransmission method of the base station. This makes it possible toprovide a base station compliant with 3 GPP-LTE.

Hereinafter base station 100 according to the present embodiment will bedescribed in detail. However, a 2-Tx base station exists in the vicinityof base station 100 at such a distance that no interference occursbetween cells. Since a terminal can move between the cell of basestation 100 and the cell of the 2-Tx base station, the terminal needs tobe able to communicate with one of the base stations seamlessly.

Furthermore, base station 100 normally operates as a virtual 2-Tx basestation that carries out virtual antenna transmission and operates as a4-Tx base station that carries out antenna port transmission for aterminal that requests antenna port transmission.

FIG. 4 illustrates a configuration of base station 100 that operates asa virtual 2-Tx base station that carries out virtual antennatransmission.

In base station 100 shown in FIG. 4, encoding section 101 encodestransmission data.

Modulation section 102 modulates the encoded data.

Mapping section 103 maps modulated data signals to virtual antenna 0 andvirtual antenna 1 respectively. The data signal mapped to virtualantenna 0 is S₀ and the data signal mapped to virtual antenna 1 is S₁.Furthermore, the RS transmitted from virtual antenna 0 is R0 and the RStransmitted from virtual antenna 1 is R1.

Inversion section 104 inverts the sign of S₀ transmitted from antennaport 2 between odd-numbered slots and even-numbered slots insynchronization with inversion section 105.

Inversion section 105 inverts the sign of R0 transmitted from antennaport 2 between odd-numbered slots and even-numbered slots insynchronization with inversion section 104.

IFFT (Inverse Fast Fourier Transform) section 106 performs an IFFT on S₀and R0 to generate an OFDM symbol.

CP (Cyclic Prefix) adding section 107 adds the same signal as that ofthe tail part of the OFDM symbol to the beginning of that OFDM symbol asa CP.

Radio transmitting section 108 performs transmission processing such asD/A conversion, amplification and up-conversion on the OFDM symbol witha CP and transmits the OFDM symbol from antenna port 0.

CDD generation section 109 generates CDD for S₀ and R0.

IFFT section 110 performs an IFFT on S₀ and R0 to generate an OFDMsymbol.

CP adding section 111 adds the same signal as that of the tail part ofthe OFDM symbol to the beginning of that OFDM symbol as a CP.

Radio transmitting section 112 performs transmission processing such asD/A conversion, amplification and up-conversion on the OFDM symbol witha CP and transmits the OFDM symbol from antenna port 2.

Inversion section 113 inverts the sign of S₁ transmitted from antennaport 3 between odd-numbered slots and even-numbered slots insynchronization with inversion section 114.

Inversion section 114 inverts the sign of R1 transmitted from antennaport 3 between odd-numbered slots and even-numbered slots insynchronization with inversion section 113.

IFFT section 115 performs an IFFT on S₁ and R1 to generate an OFDMsymbol.

CP adding section 116 adds the same signal as that of the tail part ofthe OFDM symbol to the beginning of that OFDM symbol as a CP.

Radio transmitting section 117 performs transmission processing such asD/A conversion, amplification and up-conversion on the OFDM symbol witha CP and transmits the OFDM symbol from antenna port 1.

CDD generation section 118 generates CDD for S₁ and R1.

IFFT section 119 performs an IFFT on S₁ and R1 to generate an OFDMsymbol.

CP adding section 120 adds the same signal as that of the tail part ofthe OFDM symbol to the beginning of that OFDM symbol as a CP.

Radio transmitting section 121 performs transmission processing such asD/A conversion, amplification and up-conversion on the OFDM symbol witha CP and transmits the OFDM symbol from antenna port 3.

CDD generation sections 109 and 118 are provided to suppress unnecessarybeam forming effect caused by virtual antennas.

Next, FIG. 5 illustrates a configuration of base station 100 thatoperates as a 4-Tx base station that carries out antenna porttransmission. Base station 100 shown in FIG. 5 has no inversion sections104 and 113 shown in FIG. 4. Hereinafter only the differences in FIG. 5from FIG. 4 will be described.

That is, mapping section 103 maps modulated data signals to antennaports 0, 2, 1 and 3. The data signal mapped to antenna port 0 is S₀, thedata signal mapped to antenna port 2 is S₁, the data signal mapped toantenna port 1 is S₂ and the data signal mapped to antenna port 3 is S₃.Furthermore, the RS transmitted from antenna port 0 and antenna port 2is R0 and the RS transmitted from antenna port 1 and antenna port 3 isR1.

CDD generation section 109 generates CDD to S₁ and R0.

IFFT section 110 performs an IFFT on S₁ and R0 to generate an OFDMsymbol.

IFFT section 115 performs an IFFT on S₂ and R1 to generate an OFDMsymbol.

CDD generation section 118 generates CDD to S₃ and R1.

IFFT section 119 performs an IFFT on S₃ and R1 to generate an OFDMsymbol.

Here, base station 100 (a virtual 2-Tx base station) shown in FIG. 4transmits RSs and data signals using two virtual antennas of virtualantenna 0 and virtual antenna 1. That is, suppose the basic transmissionmethod of base station 100 is virtual antenna transmission.

However, between odd-numbered slots and even-numbered slots, the sign ofthe signal from one of the two antenna ports that form one virtualantenna is inverted. In FIG. 4, the signs of R0 and S₀ at antenna port 2out of antenna port 0 and antenna port 2 that form virtual antenna 0 areinverted between odd-numbered slots and even-numbered slots. Likewise,the signs of R1 and S₁ at antenna port 3 out of antenna port 1 andantenna port 3 that form virtual antenna 1 are inverted betweenodd-numbered slots and even-numbered slots.

Since base station 100 normally operates as a virtual 2-Tx base station,RSs from base station 100 are transmitted using the same resources (thatis, the same timings and the same subcarriers) as for RSs from 2-Tx basestations that are present in the vicinity of base station 100. FIG. 6shows the arrangement of RSs in this case. However, R0′ in FIG. 6indicates resources (the timings and frequencies) whereby R0 istransmitted from antenna port 0 and antenna port 2, and R0″ indicatesresources whereby R0 is transmitted from antenna port 0 and whereby onthe other hand an RS with an inverted sign of R0 is transmitted fromantenna port 2. Likewise, R1′ in FIG. 6 indicates resources whereby R1is transmitted from antenna port 1 and antenna port 3, and R1″ indicatesresources whereby R1 is transmitted from antenna port 1 and whereby onthe other hand an RS with an inverted sign of R1 is transmitted fromantenna port 3.

Therefore, a transmission signal y from base station 100 (a virtual 2-Txbase station) is represented as follows:

$\begin{matrix}{{y = {{D\begin{pmatrix}1 & 0 \\0 & 1 \\1 & 0 \\0 & 1\end{pmatrix}}s_{2{Tx}}\mspace{14mu}{even}\mspace{14mu}{slot}}},{y = {{D\begin{pmatrix}1 & 0 \\0 & 1 \\{- 1} & 0 \\0 & {- 1}\end{pmatrix}}s_{2{Tx}}\mspace{14mu}{odd}\mspace{14mu}{slot}}}} & \lbrack 3\rbrack\end{matrix}$

As is evident from FIG. 6, the overhead required for RS transmissionwith the present embodiment is equivalent to that of a 2-Tx basestation. That is, according to the present embodiment, the throughput isnot decreased due to increased overhead.

Furthermore, since two radio transmitting sections can be used pervirtual antenna, the transmission power for an RS and the transmissionpower for a data signal become twice these transmission powers at a 2-Txbase station. Therefore, received quality at the terminal can beimproved.

Furthermore, since the same virtual antenna is used for an RS and datasignal within one RB and the base station operates as a virtual 2-Txbase station, SNR performance does not deteriorate at a high-speedterminal.

Base station 100 usually transmits both RSs and data signals to both thehigh-speed terminal and low-speed terminal using virtual antenna 0 andvirtual antenna 1.

The terminal demodulates the data signals using the received RSs on anas-is basis.

Furthermore, the terminal maintains two slots of RSs in the time domain,R0′, R0″, R1′ and R1″ maintained in this way have the followingrelationships:Received signal in R0′=signal from antenna port 0+signal from antennaport 2Received signal in R1′=signal from antenna port 1+signal from antennaport 3Received signal in R0″=signal from antenna port 0−signal from antennaport 2Received signal in R1″=signal from antenna port 1−signal from antennaport 3

Thus, the terminal can separate signals from antenna ports 0, 2, 1 and 3using the following calculations and perform channel estimation for eachantenna port.Signal from antenna port 0=received signal in R0′+received signal in R0″Signal from antenna port 2=received signal in R0′−received signal in R0″Signal from antenna port 1=received signal in R1′+received signal in R1″Signal from antenna port 3=received signal in R1′−received signal in R1″

The terminal selects one of virtual antenna transmission and antennaport transmission from the four channel estimate values of antenna ports0, 2, 1 and 3 as an optimal transmission method for base station 100 forthat terminal, and feeds back the selection result to base station 100as a transmission mode request. For example, when the correlation levelbetween the four propagation paths of antenna ports 0, 2, 1 and 3 is lowand SDM transmission can be carried out, it is generally preferable notto use virtual antenna transmission, and therefore antenna porttransmission is selected. However, since the channel estimation resultsof antenna ports 2 and 3 also include the propagation characteristics byCDD generation sections 109 and 118, even when base station 100 carriesout antenna port transmission in the configuration shown in FIG. 5, thesame CDD generation section used during virtual antenna transmission(FIG. 4) is used.

Whether virtual antenna transmission (FIG. 4) or antenna porttransmission (FIG. 5), base station 100 continues to transmit RSs whileinverting signs at antenna ports 2 and 3.

Here, suppose the output from mapping section 103 of base station 100 (a4-Tx base station) shown in FIG. 5 is:

$\begin{matrix}{s_{4{Tx}} = \begin{pmatrix}s_{0} \\s_{1} \\s_{2} \\s_{3}\end{pmatrix}} & \lbrack 4\rbrack\end{matrix}$a signal Y_(realport) transmitted from the four antenna ports is:y _(realport) =Ds _(4Tx)  [5]

Since the terminal that selects virtual antenna transmission and theterminal that selects antenna port transmission are multiplexed by OFDM,base station 100 switches between virtual antenna transmission (FIG. 4)and antenna port transmission (FIG. 5) for each terminal according torequest from each terminal. That is, base station 100 includes aswitching section that switches between virtual antenna transmission(FIG. 4) and antenna port transmission (FIG. 5) for each terminalaccording to request from each terminal, transmits RSs and data signalsin the configuration shown in FIG. 4 to a low-speed terminal thatrequests virtual antenna transmission using virtual antennas andtransmits data signals in the configuration shown in FIG. 5 to alow-speed terminal that requests antenna port transmission using antennaports.

During virtual antenna transmission, each terminal performs channelestimation using RSs on an as-is basis. On the other hand, duringantenna port transmission, each terminal separates RS for each antennaport and performs channel estimation using the separated RSs.

Here, high-speed terminals or terminals not supporting antenna porttransmission using four antenna ports may request base station 100 forvirtual antenna transmission and low-speed terminals or terminalscapable of SDM may request base station 100 for antenna porttransmission.

When requesting base station 100 for one transmission mode of virtualantenna transmission and antenna port transmission, the terminal mayalso request base station 100 to change the data signal mapping(Precoding Matrix) as well.

Upon receiving a request for virtual antenna transmission, base station100 performs mapping for two antenna ports to the virtual antenna asshown in FIG. 4, and then further performs distribution to the fourantenna ports at the virtual antennas. Therefore, the terminal requestsbase station 100 for an optimal mapping pattern out of a plurality ofmapping patterns for two antenna ports although the number of antennaports in base station 100 is four.

On the other hand, when the terminal requests base station 100 forantenna port transmission, the terminal requests base station 100 for anoptimal mapping pattern out of the plurality of mapping patterns forfour antenna ports.

The present embodiment can reduce overhead required for RS transmissioncompared to a conventional 4-Tx base station. Furthermore, since thedensity (total power) of RSs from antenna ports 2 and 3 is high, thepresent embodiment can improve the accuracy of channel estimation at theterminal. Furthermore, it is possible to increase the cell radiusthrough transmission using virtual antennas. Furthermore, since theterminal can separate RSs from virtual antennas into RSs of individualantenna ports, it is possible to request the base station for an optimaltransmission method when the base station performs SDM transmission.

A case has been described above with the present embodiment where uponreceiving a signal from base station 100 (FIG. 4) operating as a virtual2-Tx base station, the terminal is assumed to perform independentchannel estimation for each slot and change the signs of virtualantennas for each slot. However, when the terminal performs channelestimation for every n slots, the signs of virtual antennas may bechanged every n slots.

Furthermore, base station 100 in FIG. 4 and FIG. 5 need not be providedwith CDD generation sections 109 and 118.

Furthermore, the present embodiment assumes the characteristics ofvirtual antennas as:

$\begin{matrix}{{{D\begin{pmatrix}1 & 0 \\0 & 1 \\1 & 0 \\0 & 1\end{pmatrix}}\mspace{14mu}{even}\mspace{14mu}{slot}},{{D\begin{pmatrix}1 & 0 \\0 & 1 \\{- 1} & 0 \\0 & {- 1}\end{pmatrix}}\mspace{14mu}{odd}\mspace{14mu}{slot}}} & \lbrack 6\rbrack\end{matrix}$However, for example, virtual antennas having the followingcharacteristics may be used as well:

$\begin{matrix}{{{D\begin{pmatrix}1 & 0 \\1 & 0 \\0 & 1 \\0 & 1\end{pmatrix}}\mspace{14mu}{even}\mspace{14mu}{slot}},{{D\begin{pmatrix}1 & 0 \\{- 1} & 0 \\0 & 1 \\0 & {- 1}\end{pmatrix}}\mspace{14mu}{odd}\mspace{14mu}{slot}}} & \lbrack 7\rbrack\end{matrix}$

In this case, the terminal separates signals from antenna ports 0, 1, 2and 3 using the following calculations:Signal from antenna port 0=received signal in R0′+received signal in R0″Signal from antenna port 1=received signal in R0′−received signal in R0″Signal from antenna port 2=received signal in R1′+received signal in R1″Signal from antenna port 3=received signal in R1′−received signal in R1″

That is, when two vectors in even-numbered slots and odd-numbered slotseach made up of column components are extracted from the above 4×2matrix expressing the characteristics of virtual antennas, these fourvectors in total need to be orthogonal to each other. In more generalterms, signals from individual antenna ports need to have beenmultiplexed so that the terminal can separate between the propagationpaths of a plurality of antenna ports through which signals aretransmitted at the same time and using the same frequency using RSsreceived a plurality of times.

Furthermore, the present embodiment assumes that the signs of virtualantennas are changed (inverted) per slot in the time domain, and, on theother hand, the signs of virtual antennas are fixed in the frequencydomain. However, the signs of virtual antennas may also be changed(inverted) per RB in the frequency domain, and, on the other hand, thesigns of virtual antenna may be fixed in the time domain. In this case,since the characteristics of virtual antennas used for RS transmissiondo not change in the time domain, RSs received by a terminal areaveraged over a plurality of slots, so that the accuracy of channelestimation at the terminal can be improved. However, in this case, sincethe robustness against frequency selective fading decreases, the mode inwhich the signs of virtual antennas in the time domain are changed andthe mode in which the signs of virtual antennas in the frequency domainare changed may be switched adaptively according to the state of thepropagation path.

A case has been described above with the present embodiment where thebase station forms one virtual antenna with two antenna ports. However,the number of antenna ports to form one virtual antenna with, is notlimited to two in the present invention. For example, the base stationmay form one virtual antenna with four antenna ports. However, in orderfor a terminal to separate between the propagation paths of individualantenna ports, when one virtual antenna is formed with two antennaports, RSs of two slots (e.g. R0′ and R0″ shown in FIG. 6) arenecessary, whereas, when one virtual antenna is formed with four antennaports, four slots of RSs are necessary. For example, a base station thatforms one virtual antenna with four antenna ports transmits R0′, R0″,R0′″ and R0″″ for which the signs of a virtual antenna is changed perslot in the time domain. A terminal separates between the signals ofindividual antenna ports using R0′, R0″, R0′″ and R0″″ and performschannel estimation per antenna port.

Furthermore, a base station may adaptively change the number of antennaports to form one virtual antenna with, according to the situation of aterminal. That is, when a base station is provided with four antennaports, the base station may switch between the operation for forming onevirtual antenna with two antenna ports and the operation for forming onevirtual antenna with four antenna ports according to the situation ofthe terminal as shown with the present embodiment.

For example, when a terminal is moving at high speed and the variationof the propagation path in the time domain direction is significant, thestate of the propagation path changes while four RSs are beingtransmitted in the time domain direction and the terminal may not beable to separate signals correctly. Therefore, the base station mayswitch the number of antenna ports to form the virtual antenna with,according to the moving speed of a terminal. For example, a base stationforms one virtual antenna with two antenna ports for a terminal movingat high speed and arranges two RS's with changed signs in two slots. Onthe other hand, the base station forms one virtual antenna with fourantenna ports for a terminal moving at low speed and arranges four RSswith changed signs in four slots. In this way, it is possible to realizeoptimal operations according to the moving speed of the terminal whileminimizing the overhead of RSs.

Embodiment 2

The present embodiment differs from Embodiment 1 in that RS and BCHsignal are regularly transmitted by virtual antenna transmission througha BCH for indicating the number of antenna ports.

Therefore, according to the present embodiment, a terminal can provide adiversity effect through a BCH irrespective of the number of antennaports in a base station. Furthermore, a terminal separates between thepropagation paths of two antenna ports and performs channel estimation,and a terminal can thereby optimize transmission weights of the basestation after receiving a BCH signal.

Here, BCH signals having the same information are repeatedly transmittedthrough the BCH. Furthermore, the transmission of BCH signals continuesby regularly occupying part of the frequency band. Furthermore, BCHsignals are received by all terminals. Furthermore, upon receiving a BCHsignal, the number of antenna ports in a base station is unknown to theterminal.

Thus, the present embodiment adopts a common RS arrangement independentof the number of antenna ports, thereby reducing the reception load on aterminal and providing diversity gain equivalent to SFBC(Space-Frequency Block Coding).

Hereinafter transmission of a BCH signal according to the presentembodiment will be described.

According to a 3GPP-LTE procedure, a terminal acquires the SCH(Synchronization CHannel) at the start of communication with a basestation, establishes timing synchronization with the base station andthen receives a BCH signal. FIG. 7 shows the arrangement of the BCH andSCH in the time domain according to 3GPP-LTE. One slot in FIG. 7corresponds to one RB time.

Furthermore, FIG. 8 illustrates an arrangement of data channels and theBCH and SCH control channels in the frequency domain according to3GPP-LTE. These control channel signals are transmitted from the basestation using 72 subcarriers=6 RBs.

When transmitting an RS, BCH signal and SCH signal in a frequency bandincluding the BCH and SCH shown in FIG. 8, the 2-Tx base stationaccording to the present embodiment handles two antenna ports as onevirtual antenna. However, the 2-Tx base station inverts the sign of thesignal from antenna port 1 per RB in the time domain. Furthermore, the2-Tx base station operates as a normal 2-Tx base station (FIG. 1A) in afrequency band not including the BCH and SCH.

FIG. 9 illustrates a configuration of base station 200 according to thepresent embodiment.

In base station 200 shown in FIG. 9, encoding section 201 encodestransmission data (data channel).

Modulation section 202 modulates the encoded data.

Mapping section 203 maps modulated data signals to antenna port 0 andantenna port 1. The data signal mapped to antenna port 0 is S₀ and thedata signal mapped to antenna port 1 is S₁. Furthermore, RS added to S₀and transmitted from antenna port 0 is R0 and RS added to S₁ andtransmitted from antenna port 1 is R1.

On the other hand, encoding section 204 encodes BCH data (BCH).

Modulation section 205 modulates the encoded BCH data.

Mapping section 206 maps the modulated BCH data signal to virtualantenna 0 formed with antenna port 0 and antenna port 1. The BCH datasignal mapped to virtual antenna 0 is B₀.

Inversion section 207 inverts the sign of B₀ transmitted from antennaport 0 between odd-numbered slots and even-numbered slots insynchronization with inversion section 208.

inversion section 208 inverts the sign of R0 added to B₀ and transmittedfrom antenna port 0 between odd-numbered slots and even-numbered slotsin synchronization with inversion section 207.

CDD generation section 209 generates CDD for B₀ and R0.

IFFT section 210 performs an IFFT on S₀, R0 and B₀, R0 to generate anOFDM symbol.

CP adding section 211 adds the same signal as that of the tail part ofthe OFDM symbol to the beginning of that OFDM symbol as a CP.

Radio transmitting section 212 performs transmission processing such asD/A conversion, amplification and up-conversion on the OFDM symbol witha CP and transmits the OFDM symbol from antenna port 0.

IFFT section 213 performs an IFFT on S₁, R1 and B₀, R0 to generate anOFDM symbol.

CP adding section 214 adds the same signal as the tail part of the OFDMsymbol to the beginning of that OFDM symbol as a CP.

Radio transmitting section 215 performs transmission processing such asD/A conversion, amplification and up-conversion on the OFDM symbol witha CP and transmits the OFDM symbol from antenna port 1.

Thus, antenna port 0 and antenna port 1 function as virtual antenna 0 onthe BCH.

CDD generation section 209 is provided to suppress unnecessary beamforming effect caused by virtual antennas.

Next, FIG. 10A and FIG. 10B illustrate the arrangement of RSs accordingto the present embodiment. FIG. 10A illustrates an arrangement of RSs inthe frequency band not including the BCH, that is, the arrangement ofRSs in data channel bands and FIG. 10B illustrates the arrangement ofRSs in the frequency band including the BCH. Furthermore, FIG. 11illustrates an arrangement of RSs at a conventional 1-Tx base station.R0′ in FIG. 10B indicates resources whereby R0 is transmitted fromantenna port 0 and antenna port 1, and R0″ indicates resources wherebyR0 is transmitted from antenna port 0 and whereby on the other hand anRS with an inverted sign of R0 is transmitted from antenna port 1.

Thus, base station 200 adopts an RS arrangement (FIG. 10B) in thefrequency band including the BCH identical to the RS arrangement (FIG.11) at the 1-Tx base station.

By contrast, the terminal receives SCH (FIG. 7) transmitted from thebase station and acquires synchronization. Since this SCH is a sequencethat is known to the terminal in advance, the terminal can acquiresynchronization by finding the cross-correlation between the prestored,known SCH sequence and the received signal sequence. SCH is transmittedvia the virtual antenna in the same way as BCH.

After acquiring synchronization, the terminal receives R0′ or R0″,performs channel estimation and decodes the BCH sequence.

Here, base station 200 synchronizes the sign of the BCH with the sign ofthe virtual antenna used to transmit R0′ (or R0″). Therefore, theterminal needs not judge whether the received signal is R0′ or R0″.

Furthermore, base station 200 inverts signs between the sign of thevirtual antenna used in the first slot of the first subframe and thesigns of virtual antennas used in the first slot of the sixth subframein FIG. 7, and therefore can receive the BCH and the SCH throughdiversity effect even in areas where the BCH and the SCH normally do notarrive.

After receiving BCH, the terminal judges the number of antenna ports ina base station based on the BCH data and judges the RS transmissionmethod in the frequency band in which data is transmitted according tothe number of antenna ports.

According to the present embodiment, in a communication system in whicha 1-Tx base station and a 2-Tx base station are both present, theterminal can receive BCH using a reception method that is common betweenthe 1-Tx base station and the 2-Tx base station. Therefore, the terminalcan receive BCH with no errors without knowing the number of antennaports in a base station. That is, diversity transmission by a 2-Tx basestation through a BCH is made possible.

Furthermore, when a BCH is transmitted by handling two antenna ports asone virtual antenna using a method similar to the conventional one, RSis also transmitted via the virtual antenna, and therefore the terminalcannot separate between the propagation paths of the two antenna ports.By contrast, according to the present embodiment, the base stationtransmits RS while changing (inverting) the signs of virtual antennas,and therefore the terminal can separate between the propagation paths ofthe two antenna ports.

Furthermore, the present embodiment limits the frequency band usingvirtual antennas to some bands including the BCH. Therefore, in afrequency band not using virtual antennas (e.g. the frequency band ofthe data channel), it is possible to improve the accuracy of channelestimation by averaging RSs received at a terminal in the time domain.

Furthermore, according to the present embodiment, in a communicationsystem in which a plurality of base stations with different numbers ofantenna ports are present, a terminal can receive a diversity effect foreach base station while realizing a common RS arrangement, so that it ispossible to increase the degree of freedom in the design of thecommunication system.

Transmission of control information other than the BCH can also becarried out in the same way as the transmission of the BCH.

Furthermore, when the SCH is transmitted via a virtual antenna in thesame way as the BCH, it is preferable to generate CDD for the BCH aloneand not generate CDD for the SCH. This eliminates additional arrivaltime differences by CDD among a plurality of antenna ports, so that itis possible to improve the performance of acquiring synchronizationusing the SCH.

Diversity effect is compared between the BCH transmission according tothe present embodiment and the BCH transmission using SFBC as follows.Here, suppose the number of transmitting antenna ports in the basestation is 2 and the number of receiving antenna ports of the terminalis 1, the characteristics of the propagation path from antenna port 0 ofthe base station to the antenna port of the terminal is h₀(f) and thecharacteristics of the propagation path from antenna port 1 of the basestation to the antenna port of the terminal is h₁(f).

In BCH transmission using SFBC, the received power of the terminal isgiven by:

$\begin{matrix}{\sum\limits_{f}\{ {{h_{0}^{2}(f)} + {h_{1}^{2}(f)}} \}} & \lbrack 8\rbrack\end{matrix}$

On the other hand, in BCH transmission according to the presentembodiment, the received power of the terminal is given by:

$\begin{matrix}{{\sum\limits_{f}{\{ {{h_{0}(f)} + {{\mathbb{e}}^{{- {j2\pi}}\; f\;\delta}{h_{1}(f)}}} \}^{2}\mspace{14mu}\ldots\mspace{14mu}{even}\mspace{14mu}{slot}}},{\sum\limits_{f}{\{ {{h_{0}(f)} - {{\mathbb{e}}^{{- {j2\pi}}\; f\;\delta}{h_{1}(f)}}} \}^{2}\mspace{14mu}\ldots\mspace{14mu}{odd}\mspace{14mu}{slot}}},} & \lbrack 9\rbrack\end{matrix}$where, e^(−j2πfδ) is the CDD component.

Therefore, according to the present embodiment, the average receivedpower of BCH at the terminal is:

$\begin{matrix}{\frac{{\sum\limits_{f}\{ {{h_{0}(f)} + {{\mathbb{e}}^{{- {j2\pi}}\; f\;\delta}{h_{1}(f)}}} \}^{2}} + {\sum\limits_{f}\{ {{h_{0}(f)} - {{\mathbb{e}}^{{- {j2\pi}}\; f\;\delta}{h_{1}(f)}}} \}^{2}}}{2} = {{\sum\limits_{f}\{ {{h_{0}^{2}(f)} + {{{\mathbb{e}}^{{- {j2\pi}}\; f\;\delta}}^{2}{h_{1}^{2}(f)}}} \}} = {\sum\limits_{f}\{ {{h_{0}^{2}(f)} + {h_{1}^{2}(f)}} \}}}} & \lbrack 10\rbrack\end{matrix}$which is equal to the average received power of SFBC.

Furthermore, the maximum received power of BCH transmission according tothe present embodiment is greater than the maximum received power of BCHtransmission by SFBC. Therefore, according to BCH transmission accordingto the present embodiment, the maximum outreach of BCH can be greaterthan that of BCH transmission by SFBC. Therefore, the present embodimentcan provide diversity effect exceeding SFBC for information transmitteda plurality of times repeatedly as with BCH.

Embodiment 3

The present embodiment is different from Embodiment 1 in that a basestation with eight antenna ports (8-Tx base station) transmits datasignals.

The number of antenna ports in a base station according to 3GPP-LTE isfour at a maximum. Therefore, a 3GPP-LTE-compliant terminal candemodulate data and measure quality of a downlink signal using the RStransmitted from a base station provided with a maximum of four antennaports (a 4-Tx base station).

By contrast, LTE-advanced, which is a developed version of 3GPP-LTE, isstudying a base station provided with a maximum of eight antenna ports(8-Tx base station). However, even LTE-advanced needs to provide a3GPP-LTE-compliant base station so as to allow terminals supporting only3GPP-LTE-compliant base stations (a 4-Tx base station) to communicate.In other words, in a communication system in which a 4-Tx base station(a base station according to 3GPP-LTE) and an 8-Tx base station (a basestation according to LTE-advanced) are both present, a terminal thatsupports 4-Tx base stations alone (hereinafter referred to as “LTEterminal”) and a terminal also supporting 8-Tx base stations(hereinafter referred to as an “LTE+ terminal”) need to be able tocommunicate with each other in the same frequency band.

Therefore, an 8-Tx base station according to the present embodimenttransmits RS and data signals to LTE terminals using four virtualantennas, each made up of two antenna ports (virtual antennatransmission). Furthermore, the 8-Tx base station according to thepresent embodiment transmits RS and data signals to LTE+ terminalssuitable for transmission of data signals using eight antenna ports,without using virtual antennas yet using eight antenna ports (antennaport transmission).

However, during antenna port transmission, the 8-Tx base stationaccording to the present embodiment inverts the sign of one of the twoRS which are arranged only in the frequency band in which data signalsare transmitted via antenna ports, and which are transmitted in commonfrom two antenna ports forming one virtual antenna.

Base station 300 according to the present embodiment will be describedin detail.

FIG. 12 illustrates a configuration of base station 300 that operates asa virtual 4-Tx base station that carries out virtual antennatransmission.

In base station 300 shown in FIG. 12, encoding section 301 encodestransmission data.

Modulation section 302 modulates the encoded data.

Mapping section 303 maps modulated data signals to virtual antenna 0,virtual antenna 1, virtual antenna 2 and virtual antenna 3. The datasignal mapped to virtual antenna 0 is S₀, the data signal mapped tovirtual antenna 1 is S₁, the data signal mapped to virtual antenna 2 isS₂ and the data signal mapped to virtual antenna 3 is S₃. Furthermore,the RS transmitted from virtual antenna 0 is R0, the RS transmitted fromvirtual antenna 1 is R1, the RS transmitted from virtual antenna 2 is R2and the RS transmitted from virtual antenna 3 is R3.

IFFT section 304 performs an IFFT on S₀ and R0, to generate an OFDMsymbol.

CP adding section 305 adds the same signal as that of the tail part ofthe OFDM symbol to the beginning of that OFDM symbol as a CP.

Radio transmitting section 306 performs transmission processing such asD/A conversion, amplification and up-conversion on the OFDM symbol witha CP and transmits the OFDM symbol from antenna port 0.

CDD generation section 307 generates CDD for S₀ and R0.

IFFT section 308 performs an IFFT on S₀ and R0, to generate an OFDMsymbol.

CP adding section 309 adds the same signal as that of the tail part ofthe OFDM symbol to the beginning of that OFDM symbol as a CP.

Radio transmitting section 310 performs transmission processing such asD/A conversion, amplification and up-conversion on the OFDM symbol witha CP and transmits the OFDM symbol from antenna port 1.

IFFT section 311 performs an IFFT on S₁ and R1, to generate an OFDMsymbol.

CP adding section 312 adds the same signal as that of the tail part ofthe OFDM symbol to the beginning of that OFDM symbol as a CP.

Radio transmitting section 313 performs transmission processing such asD/A conversion, amplification and up-conversion on the OFDM symbol witha CP and transmits the OFDM symbol from antenna port 2.

CDD generation section 314 generates CDD for S₁ and R1.

IFFT section 315 performs an IFFT on S₁ and R1, to generate an OFDMsymbol.

CP adding section 316 adds the same signal as that of the tail part ofthe OFDM symbol to the beginning of that OFDM symbol as a CP.

Radio transmitting section 317 performs transmission processing such asD/A conversion, amplification and to up-conversion on the OFDM symbolwith a CP and transmits the OFDM symbol from antenna port 3.

IFFT section 318 performs an IFFT on S₂ and R2, to generate an OFDMsymbol.

CP adding section 319 adds the same signal as that of the tail part ofthe OFDM symbol to the beginning of that OFDM symbol as a CP.

Radio transmitting section 320 performs transmission processing such asD/A conversion, amplification and up-conversion on the OFDM symbol witha CP and transmits the OFDM symbol from antenna port 4.

CDD generation section 321 generates CDD for S₂ and R2.

IFFT section 322 performs an IFFT on S₂ and R2, to generate an OFDMsymbol.

CP adding section 323 adds the same signal as that of the tail part ofthe OFDM symbol to the beginning of that OFDM symbol as a CP.

Radio transmitting section 324 performs transmission processing such asD/A conversion, amplification and up-conversion on the OFDM symbol witha CP and transmits the OFDM symbol from antenna port 5.

IFFT section 325 performs an IFFT on S₃ and R3, to generate an OFDMsymbol.

CP adding section 326 adds the same signal as that of the tail part ofthe OFDM symbol to the beginning of that OFDM symbol as a CP.

Radio transmitting section 327 performs transmission processing such asD/A conversion, amplification and up-conversion on the OFDM symbol witha CP and transmits the OFDM symbol from antenna port 6.

CDD generation section 328 generates CDD for S₃ and R3.

IFFT section 329 performs an IFFT on S₃ and R3, to generate an OFDMsymbol.

CP adding section 330 adds the same signal as that of the tail part ofthe OFDM symbol to the beginning of that OFDM symbol as a CP.

Radio transmitting section 331 performs transmission processing such asD/A conversion, amplification and up-conversion on the OFDM symbol witha CP and transmits the OFDM symbol from antenna port 7.

CDD generation sections 307, 314, 321 and 328 are provided to suppressunnecessary beam forming effect caused by virtual antennas.

Next, FIG. 13 illustrates a configuration of base station 300 operatingas an 8-Tx base station that carries out antenna port transmission. InFIG. 13, only differences from FIG. 12 will be described.

That is, mapping section 303 maps modulated data signals S₀ to S₇ toantenna ports 0 to 7 respectively. Furthermore, the RSs transmitted fromantenna port 0 and antenna port 1 are R0 and R4, the RSs transmittedfrom antenna port 2 and antenna port 3 are R1 and R5, the RSstransmitted from antenna port 4 and antenna port 5 are R2 and R6 and theRSs transmitted from antenna port 6 and antenna port 7 are R3 and R7.That is, base station 300 that performs antenna port transmission (8-Txbase station) further transmits R4 to R7 in addition to RSs transmittedthrough the virtual antenna transmission shown in FIG. 12.

IFFT section 304 performs an IFFT on S₀, R0 and R4, to generate an OFDMsymbol.

Inversion section 332 inverts the sign of R4 transmitted from antennaport 1.

CDD generation section 307 generates CDD for S₁, R0 and R4.

IFFT section 308 performs an IFFT on S₁, R0 and R4, to generate an OFDMsymbol.

IFFT section 311 performs an IFFT on S₂, R1 and R5, to generate an OFDMsymbol.

Inversion section 333 inverts the sign of R5 transmitted from antennaport 3.

CDD generation section 314 generates CDD for S₃, R1 and R5.

IFFT section 315 performs an IFFT on S₃, R1 and R5, to generate an OFDMsymbol.

IFFT section 318 performs an IFFT on S₄, R2 and R6, to generate an OFDMsymbol.

Inversion section 334 inverts the sign of R6 transmitted from antennaport 5.

CDD generation section 321 generates CDD for S₅, R2 and R6.

IFFT section 322 performs an IFFT on S₅, R2 and R6, to generate an OFDMsymbol.

IFFT section 325 performs an IFFT on S₆, R3 and R7, to generate an OFDMsymbol.

Inversion section 335 inverts the sign of R7 transmitted from antennaport 7.

CDD generation section 328 generates CDD for S₇, R3 and R7.

IFFT section 329 performs an IFFT on S₇, R3 and R7, to generate an OFDMsymbol.

Here, for R4 to R7 only transmitted during antenna port transmission,base station 300 shown in FIG. 13 inverts the sign of the RS transmittedfrom one of the two antenna ports forming one virtual antenna in FIG.12. That is, base station 300 inverts the sign of R4 transmitted fromantenna port 1 out of antenna port 0 and antenna port 1 forming virtualantenna 0 in FIG. 12. Likewise, base station 300 inverts the sign of R5transmitted from antenna port 3 out of antenna port 2 and antenna port 3forming virtual antenna 1 in FIG. 12. The same applies to virtualantennas 2 and 3 (antenna ports 4 to 7).

Next, FIG. 14 illustrates an arrangement of RSs according to the presentembodiment. R0′ in FIG. 14 indicates resources whereby R0 is transmittedfrom antenna port 0 and antenna port 1, R1′ indicates resources wherebyR1 is transmitted from antenna port 2 and antenna port 3, R2′ indicatesresources whereby R2 is transmitted from antenna port 4 and antenna port5 and R3′ indicates resources whereby R3 is transmitted from antennaport 6 and antenna port 7. Furthermore, R4′ indicates resources wherebyR4 is transmitted from antenna port 0 and whereby on the other hand anRS with an inverted sign of R4 is transmitted from antenna port 1, R5′indicates resources whereby R5 is transmitted from antenna port 2 andwhereby on the other hand an RS with an inverted sign of R5 istransmitted from antenna port 3, R6′ indicates resources whereby R6 istransmitted from antenna port 4 and whereby on the other hand an RS withan inverted sign of R6 is transmitted from antenna port 5, and R7′indicates resources whereby R7 is transmitted from antenna port 6 andwhereby on the other hand an RS with an inverted sign of R7 istransmitted from antenna port 7.

Furthermore, as shown in FIG. 14, base station 300 divides the entirefrequency band (subcarrier numbers 0 to 23) into a transmission band(subcarrier numbers 0 to 11, hereinafter referred to as the “4-RStransmission band”) where data signals directed to LTE terminalssupporting only 4-Tx base stations (or terminals that receive downlinkdata signals in the 4-RS transmission band out of LTE+ terminals) arearranged and a transmission band (subcarrier numbers 12 to 23,hereinafter referred to as the “8-RS transmission band”) where datasignals directed to LTE+ terminals also supporting 8-Tx base stationsare arranged. Base station 300 may also broadcast the result of divisionof the frequency band to the LTE+ terminals or indicate informationshowing that eight RSs are transmitted in the 8-RS transmission bandonly to terminals whose data signals are allocated to the 8-RStransmission band using a downlink control signal (e.g. PDCCH).

R0′ to R3′ shown in FIG. 14 are transmitted using the same resources(the same timings and the same subcarriers) as those of R0 to R3 (FIG.2B) from a 4-Tx base station according to 3GPP-LTE. Furthermore, R0′ toR3′ are arranged in all frequency bands (subcarrier numbers 0 to 23shown in FIG. 14). Furthermore, R4 to R7 are further arranged in the8-RS transmission band (subcarrier numbers 12 to 23) shown in FIG. 14 inaddition to R0 to R3.

When base station 300 operates as a virtual 4-Tx base station, RSs frombase station 300 are transmitted through R0′ to R3′ arranged in the 4-RStransmission band shown in FIG. 14. That is, overhead required for RStransmission during virtual transmission is equivalent to that of a 4-Txbase station. Furthermore, R0′ shown in FIG. 14 is transmitted fromantenna port 0 and antenna port 1 at the same time and using the samefrequency, R1′ is transmitted from antenna port 2 and antenna port 3 atthe same time and using the same frequency, R2′ is transmitted fromantenna port 4 and antenna port 5 at the same time and using the samefrequency and R3′ is transmitted from antenna port 6 and antenna port 7at the same time and using the same frequency. Therefore, since RSs canbe transmitted using two radio transmitting sections per virtualantenna, the transmission power of RS and the transmission power of adata signal become twice these transmission powers at a 4-Tx basestation. Therefore, the received quality at a terminal can be improved.

On the other hand, when base station 300 operates as an 8-Tx basestation, RSs from base station 300 are transmitted through R0 to R3arranged in the 8-RS transmission band shown in FIG. 14 and R4 to R7arranged only in the 8-RS transmission band. However, as shown in FIG.13, although R4′ is transmitted from antenna port 0 and antenna port 1at the same time and using the same frequency, the sign of R4transmitted from antenna port 1 is inverted with respect to R4transmitted from antenna port 0. The same applies to R5′, R6′ and R7′.

That is, in base station 300 shown in FIG. 13, this is equivalent to(1, 1) being assigned to R0, R1, R2 and R3 as a virtual antenna weight,and (1, −1), which is orthogonal to (1, 1) being assigned to R4, R5, R6and R7, as a virtual antenna weight.

Next, transmission data will be described. In base station 300 shown inFIG. 12, transmission data transmitted in the 4-RS transmission band ismapped to four virtual antennas 0 to 3 by mapping section 303. The samevirtual antenna weight as the virtual antenna weight assigned to R0, R1,R2 and R3 is applied to the transmission data mapped to each virtualantenna. On the other hand, in base station 300 shown in FIG. 13,transmission data transmitted in the 8-RS transmission band is directlymapped to eight antenna ports by mapping section 303. However, sincetransmission data transmitted in the 8-RS transmission band is mapped tothe eight antenna ports in mapping section 303, no virtual antennaweight is applied.

Next, the operations of a terminal will be described.

For example, when an LTE terminal measures the downlink power(measurement) used in the case of handover or a new cell search, the LTEterminal uses R0′ to R3′. That is, an LTE terminal measures the receivedpower of R0′ to R3′ as the signal strength of virtual antennas 0 to 3.The LTE terminal then feeds back the measurement result to base station300. Here, the LTE terminal needs not judge whether the RS used formeasurement is the RS transmitted via the four antenna ports of the 4-Txbase station or the RS transmitted by the 8-Tx base station usingvirtual antennas. That is, an LTE terminal can measure downlink powerwithout distinguishing between a 4-Tx base station and an 8-Tx basestation. Furthermore, since R0′ to R3′ used for measurement aretransmitted using virtual antennas, formed with two antenna ports each,in base station 300, the density (total power) of RSs increases in theLTE terminal, so that it is possible to perform accurate measurement.

Furthermore, when an LTE terminal (or a terminal out of LTE+ terminalsthat receives a downlink data signal in the 4-RS transmission band)receives a downlink data signal in the 4-RS band shown in FIG. 14, theLTE terminal performs channel estimation per antenna port using R0′ toR3′. The LTE terminal then receives data signals transmitted from basestation 300 using four channel estimate values and an antenna portmapping pattern for 4 antenna ports indicated beforehand from basestation 300. Here, since base station 300 assigns weights of the virtualantennas to virtual antennas 0 to 3, the LTE terminal can receivedownlink data signals without taking into account the fact that thenumber of antenna ports in base station 300 is eight.

Furthermore, when an LTE+ terminal receives a downlink data signal inthe 8-RS transmission band shown in FIG. 14, the LTE+ terminal separatesthe signals from antenna ports 0 to 7 using the following calculationsand performs channel estimation per antenna port.Signal from antenna port 0=(received signal in R0′+received signal inR4′)/2Signal from antenna port 1=(received signal in R0′−received signal inR4′)/2Signal from antenna port 2=(received signal in R1′+received signal inR5′)/2Signal from antenna port 3=(received signal in R1′−received signal inR5′)/2Signal from antenna port 4=(received signal in R2′+received signal inR6′)/2Signal from antenna port 5=(received signal in R2′−received signal inR6′)/2Signal from antenna port 6=(received signal in R3′+received signal inR7′)/2Signal from antenna port 7=(received signal in R3′−received signal inR7′)/2

Thus, an LTE+ terminal receives a downlink data signal transmitted frombase station 300 using eight channel estimate values of antenna ports 0to 7 and an antenna port mapping pattern for eight antenna ports.

Furthermore, the LTE+ terminal feeds back eight channel estimate valuesof antenna ports 0 to 7 to base station 300 over an uplink. Base station300 determines an antenna port mapping pattern to be applied to downlinkdata directed to the LTE+terminal for the next and subsequenttransmissions based on feedback information.

Since the LTE terminal to which virtual antenna transmission is appliedand the LTE+ terminal to which antenna port transmission is applied aremultiplexed by OFDM, base station 300 switches between virtual antennatransmission (FIG. 12) and antenna port transmission (FIG. 13) for eachterminal. That is, base station 300 has a switching section thatswitches between virtual antenna transmission (FIG. 12) and antenna porttransmission (FIG. 13) for each terminal. Base station 300 then maps RSsand data signals in the configuration shown in FIG. 12 to the 4-RStransmission band shown in FIG. 14 for the LTE terminal to which virtualantenna transmission is applied, and carries out virtual antennatransmission. On the other hand, base station 300 maps RSs and datasignals in the configuration shown in FIG. 13 to the 8-RS transmissionband shown in FIG. 14 for the LTE+ terminal to which antenna porttransmission is applied and carries out antenna port transmission.

The LTE terminal then performs channel estimation using the RSstransmitted through the virtual antennas on an as-is basis. On the otherhand, the LTE+ terminal separates RSs transmitted through the antennaports for each antenna port and performs channel estimation using theseparated RSs.

With the present embodiment, the 8-Tx base station uses virtual antennasfor the LTE terminal, and thereby carries out virtual antennatransmission using all the eight antenna ports. Therefore, the eightantenna ports can be used effectively. That is, the LTE terminalreceives the same RS from two antenna ports forming a virtual antenna,so that it is possible to improve received quality. Furthermore, throughtransmission using virtual antennas, the LTE terminal can performmeasurement in the case of handover or a new cell search withoutdistinguishing the number of antenna ports in a base station.

Furthermore, with the present embodiment, an 8-Tx base station transmitsdata signals using eight antenna ports without using virtual antennas tothe LTE+ terminal which is suitable for transmission of data signalsusing eight antenna ports. Here, the 8-Tx base station additionallyarranges RSs only in the frequency band in which data to be transmittedto LTE+ terminals supporting reception of 8 RSs is arranged, so that itis possible to minimize the overhead of RSs. Furthermore, an LTE+terminal can separate the received RSs into the RSs of the eight antennaports. This allows an LTE terminal that desires virtual antennatransmission and an LTE+ terminal that desires antenna port transmissionto be both present within a cell covered by a base station.

Base station 300 in FIG. 12 and FIG. 13 needs not be provided with a CDDgeneration section.

Embodiments of the present invention have been described so far.

A terminal may also be referred to as “UE,” a base station apparatus mayalso be referred to as a “Node B” and a subcarrier may also be referredto as a “tone.”

Furthermore, a CP may also be referred to as a “guard interval (GI)”.

Furthermore, the method of transformation between the frequency domainand the time domain is not limited to the IFFT and FFT.

Furthermore, the present invention is applicable not only to basestations and terminals but is applicable to all radio communicationapparatuses.

Also, although cases have been described with the above embodiments asexamples where the present invention is configured by hardware, thepresent invention can also be realized by software.

Each function block employed in the description of each of theaforementioned embodiments may typically be implemented as an LSIconstituted by an integrated circuit. These may be individual chips orpartially or totally contained on a single chip. “LSI” is adopted herebut this may also be referred to as “IC,” “system LSI,” “super LSI,” or“ultra LSI” depending on differing extents of integration.

Further, the method of circuit integration is not limited to LSI's, andimplementation using dedicated circuitry or general purpose processorsis also possible. After LSI manufacture, utilization of a programmableFPGA (Field Programmable Gate Array) or a reconfigurable processor whereconnections and settings of circuit cells within an LSI can bereconfigured is also possible.

Further, if integrated circuit technology comes out to replace LSI's asa result of the advancement of semiconductor technology or a derivativeother technology, it is naturally also possible to carry out functionblock integration using this technology. Application of biotechnology isalso possible.

INDUSTRIAL APPLICABILITY

The present invention is applicable to a mobile communication system orthe like.

The invention claimed is:
 1. A base station apparatus comprising: aplurality of antenna ports; and a transmitter configured to transmit,via the plurality of antenna ports, first reference signals on aplurality of first resource elements, and second reference signalsdifferent from the first reference signals on a plurality of secondresource elements, a number of which in a resource block is less than anumber of the plurality of first resource elements in the resourceblock, the first reference signals being associated with mobile stationssupporting base station configurations of up to four antenna ports, andthe second reference signals being associated with mobile stationssupporting base station configurations of up to eight antenna ports,wherein the transmitter is configured to transmit on one of theplurality of second resource elements and via a pair of the plurality ofantenna ports: a first reference signal of the second reference signals,which is multiplied by “1”, via a first one of the pair of antennaports; and a second reference signal of the second reference signals,which is multiplied by “−1”, via a second one of the pair of antennaports.
 2. The base station apparatus according to claim 1, wherein aphase of the second reference signal of the second reference signals isreversed relative to a phase of the first reference signal of the secondreference signals.
 3. The base station apparatus according to claim 1,wherein prior to multiplication, the first and second reference signalsof the second reference signals are identical.
 4. The base stationapparatus according to claim 1, wherein the first reference signals areused for both downlink channel quality estimation and downlink powermeasurement, and the second reference signals are used for downlinkchannel quality estimation.
 5. The base station apparatus according toclaim 1, wherein a number of resource elements for the first referencesignals transmitted per antenna port is greater than a number ofresource elements for the second reference signals transmitted perantenna port.
 6. The base station apparatus according to claim 1,wherein a number of antenna ports of the plurality of antenna portswhich are configured for a mobile station corresponds to which one orboth of the first reference signals and the second reference signals areused by the mobile station for channel quality estimation.
 7. The basestation apparatus according to claim 1, wherein, the first referencesignals are associated with mobile stations supporting base stationconfigurations of up to four antenna ports and with mobile stationssupporting base station configurations of up to eight antenna ports; andthe second reference signals are only associated with mobile stationssupporting base station configurations of up to eight antenna ports. 8.The base station apparatus according to claim 1 wherein, the mobilestations supporting base station configuration of up to four antennaports are configured to support LTE, and the mobile stations supportingbase station configurations of up to eight antenna ports are configuredto support LTE-Advanced.
 9. The base station apparatus according toclaim 6, wherein the number of antenna ports configured for the mobilestation is 4 or less in correspondence with a case where the firstreference signals are used for channel quality estimation, and thenumber of antenna ports configured for the mobile station is 8 incorrespondence with a case where the second reference signals are usedfor channel quality estimation.
 10. A transmission method performed by abase station apparatus comprising: transmitting, via a plurality ofantenna ports, a-first reference signals on a plurality of firstresource elements and second reference signals different from the firstreference signals on a plurality of a-second resource elements, a numberof which in a resource block is less than a number of the plurality offirst resource elements in the resource block, the first referencesignals being associated with mobile stations supporting base stationconfigurations of up to four antenna sorts and the second referencesignals being associated with mobile stations supporting base stationconfigurations of up to eight antenna ports, wherein the transmittingincludes transmitting on one of the second resource elements of theplurality of second resource elements and via a pair of the plurality ofantenna ports: a first one of the second reference signals, which ismultiplied by “1”, via a first one of the pair of antenna ports amongthe plurality of antenna ports; and a second one of the second referencesignals, which is multiplied by “−1”, via a second one of the pair ofantenna ports among the plurality of antenna ports.
 11. The transmissionmethod according to claim 10, wherein a phase of the second one of thesecond reference signals is reversed relative to a phase of said firstone of the second reference signals.
 12. The transmission methodaccording to claim 10, wherein the first reference signals are used forboth downlink channel quality estimation and downlink power measurement,and the second reference signals are used for downlink channel qualityestimation.
 13. The transmission method according to claim 10, wherein anumber of antenna ports of the plurality of antenna ports which areconfigured for a mobile station among the plurality of antenna ports,corresponds to which one or both of the first reference signals and thesecond reference signals are used by the mobile station for channelquality estimation.
 14. The transmission method according to claim 10,wherein a number of resource elements for the first reference signalstransmitted per antenna port is greater than a number of resourceelements for the second reference signals transmitted per antenna port.15. The transmission method according to claim 10, wherein, the firstreference signals are associated with mobile stations supporting basestation configurations of up to four antenna ports and with mobilestations supporting base station configurations of up to eight antennaports; and the second reference signals are associated with only mobilestations supporting base station configurations of up to eight antennaports.
 16. The transmission method according to claim 10, wherein, themobile stations supporting base station configurations of up to fourantenna ports are configured to support LTE; and the mobile stationssupporting base station configuration of up to eight antenna ports areconfigured to support LTE-Advanced.
 17. The transmission methodaccording to claim 13, wherein the number of antenna ports configuredfor the mobile station is 4 or less in correspondence with a case wherethe first reference signals are used for channel quality estimation, andthe number of antenna ports configured for the mobile station is 8 incorrespondence with a case where the second reference signals are usedfor channel quality estimation.
 18. A base station apparatus,comprising: a plurality of antenna ports; and a transmitter configuredto transmit, via the plurality of antenna ports, first reference signalson a plurality of first resource elements, and second reference signalsdifferent from the first reference signals on a plurality of secondresource elements, wherein a number of the plurality of second resourceelements in a resource block is less than a number of the plurality offirst resource elements in the resource block, the first referencesignals being associated with mobile stations of a first type and thesecond reference signals being associated with mobile stations of asecond type, wherein the transmitter is configured to transmit on asecond resource element and via a pair of antenna ports of the pluralityof antenna ports: a first one of the second reference signals multipliedby “1”, via a first one of the pair of antenna ports; and a second oneof the second reference signals multiplied by “−1”, via a second one ofthe pair of antenna ports.
 19. The base station of claim 18 wherein thefirst type of mobile stations support base station configurations of upto four antenna ports and the second type of mobile stations supportbase station configurations of up to eight antenna ports.
 20. The basestation of claim 18 wherein the first type of mobile stations areconfigured to support LTE and the second type of mobile stations areconfigured to support LTE-Advanced.
 21. The base station of claim 18wherein the base station is configured to use up to four antenna portsto transmit a first reference signal when the first reference signal isused for channel quality estimation and to use eight antenna ports totransmit a second reference signal when the second reference signal isused for channel quality estimation.
 22. The base station of claim 18wherein, prior to the multiplication of the second one of the secondreference signals, the first one of the second reference signals and thesecond one of the second reference signals are identical.